Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan
Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan
复制标题
DOI：
10.2138/am-2000-1004
复制
发表时间：
2000-10-01
影响因子：
3.1
通讯作者：
Maruyama, S
中科院分区：
文献类型：
--
作者：
Katayama, I;Parkinson, CD;Maruyama, S
哈萨克斯坦Kokchetav地块榴辉岩的基质单斜辉石中含有丰富的出溶石英棒。这些岩石是钻石级超高压变质岩，在P>；6 GPA和T>；1000℃时重结晶。锆石是一个极好的容器，有效地保护了峰期超高压变质相不受退变。因此，为了确定单斜辉石的出溶前成分，我们分析了榴辉岩和含金刚石黑云片麻岩中的锆石中的单斜辉石包裹体。锆石中的单斜辉石的Fe~(3+)+Al~(6+)比Al~(3+)+Al~(2+)+K超额，计算出的阳离子总量明显低于4~4个O原子。如果考虑到Ca-Eskola端元(Ca(0.5)平方(0.5)AlSi(2)O(6))，这些辉石的化学计量可以得到协调。榴辉岩中以锆石为主的单斜辉石的Ca-Eskola组分含量高达9.6%，黑云片麻岩中的单斜辉石含量高达18.2m ol%，而基质单斜辉石的含量则少得多(平均1.3m ol%)。在石英棒出溶前重新计算基质单斜辉石的组成，得到6.8摩尔%的Ca-Eskola组分，这与锆石中单斜辉石包裹体的组成一致。结果表明，峰期变质单斜辉石中的Ca-Eskola组分通过2Ca(0.5)方(0.5)AlSi(2)O(6)--&Gt；CaAl2SiO6+3SiO2的逆行反应分解，导致石英棒在基质单斜辉石中出溶。俯冲的地壳和表壳岩石在高压和高温下形成了Ca-Eskola单斜辉石。含空位的单斜辉石可能对上地幔深度俯冲板岩的物理化学性质有重要影响。
Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahigh-pressure (UHP) metamorphic rocks that recrystallized at P > 6 GPa and T > 1000 degrees C. Zircon is an excellent container, which effectively protects peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond-bearing biotite gneiss. Clinopyroxene in zircon has an excess of Fe3+ + Al-VI over Al-IV + Na + K, and calculated cation totals significantly less than 4.0 per six O atoms. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member (Ca(0.5)square(0.5)AlSi(2)O(6)) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola component, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clinopyroxene contains much less (1.3 mol%, on average). Recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6.8 mol% Ca-Eskola component, which is consistent with the composition of the clinopyroxene inclusions in zircon. We conclude that the Ca-Eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction:2 Ca(0.5)square(0.5)AlSi(2)O(6) --> CaAl2SiO6 + 3 SiO2resulting in exsolution of the quartz rods in the matrix clinopyroxene.Subducted crustal and supracrustal rocks form the Ca-Eskola clinopyroxene at high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subducted slab at upper mantle depth.